Hello Roger,
Thank you for using Apple Support Communities.
We understand that you unable to switch your iPhone out of Dark Mode. Let's see what we can do to resolve this for you. To start with, take a look at the following support article to use Dark Mode. Make sure that you have not set Dark Mode to turn on automatically.
Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support
Also, let's go to Settings > Accessibility > Display & Text Size and check if either Smart Invert or Classic Invert are enabled there. If so, if you turn those off you should be able to switch between light and dark without issue.
If those are already disabled, or the issue continues after you disabled them, you may want to consider updating your device if you're not already on iOS 14.0.1. You can find the steps for updating here: Update your iPhone, iPad, or iPod touch
Best regards.